The present invention relates to an optical disk, and more particularly preferably relates to a magneto-optical disk used as a postscript-type optical disk.
The prior art optical disk includes an optical disk (or postscript-type optical disk (WORM disk)) capable of writing predetermined information data once only, and a rewritable optical disk (or magneto-optical disk (MO disk)) capable of writing information data over and over again.
The WORM disk and the MO disk have different recording and reproducing principles and recording medium structures. Hence, a recording and reproducing apparatus that is capable of using both types of disk must use a recording laser with selectable power optimized for characteristics of each recording medium. Such an apparatus must also have a selectable reproducing equalizer characteristic.
WORM disks can use various types of recording media recording such as perforated disk, phase change disk, alloy type disk, and the like. Hence, to be able to record and reproduce any disk, the recording laser power and the reproducing equalizer characteristic of the recording and reproducing apparatus must be adjusted for the particular species of WORM disk used. Consequently, the construction of the recording and reproducing apparatus is complicated.
The ability of the WORM disk to write data only once is regarded by some as preventing tampering with the recorded data.
In practice, however, when writing information data, if a sector, which will be called an N.G. sector, with a write/read error is detected in the user recording area, a so-called alternation process is carried out. In the alternation process, information data that would otherwise be written in the N.G. sector is written in another sector instead of the N.G. sector. Consequently, the information data is, in effect, tampered with simply and undetectably. Therefore, the WORM disk can be regarded as being equivalent to the rewritable MO disk.
Further in WORM disk, flag information and other information for indicating whether or not certain sectors of the user recording area are unused or whether or not an alternation process has been carried out are written in each sector.
This means that, in a WORM disk, when searching for an empty sector by detecting whether or not certain sectors of the user recording area are unused, the flag information in each sector must be read. Also, error detection and correction must be applied to the content of the sector. The search process consequently requires considerable time.
If a certain sector of the user recording area is determined to be an N.G. sector, and an alternation process is carried out, the flag information read out for determining whether or not the sector is unused becomes uncertain. To restore certainty, the information data in the sector must be read out and error detection and correction applied. This is also unsatisfactory with respect to the time required for the process and its precision.
To solve such problems in detecting whether or not a predetermined sector of the user recording area is unused, and thus preventing overwriting a used sector, an MO disk, which is regarded as equivalent to a WORM disk may be used as a postscript-type optical disk.